\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage[english]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{lmodern}
\usepackage{graphicx}
\usepackage{wrapfig}
\usepackage{multicol}
\usepackage[authoryear,sort]{natbib}
\usepackage{geometry}
\geometry{top=3cm, bottom=3cm, left=3cm, right=3cm}